Magnetic recording (“MR”) media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of the grains of magnetic material.
A portion of a conventional longitudinal recording, thin-film, hard disk-type magnetic recording medium 1 commonly employed in computer-related applications is schematically illustrated in FIG. 1 in simplified cross-sectional view, and comprises a substantially rigid, non-magnetic metal substrate 10, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Al—Mg) alloy, having sequentially deposited or otherwise formed on a surface 10A thereof a plating layer 11, such as of amorphous nickel-phosphorus (Ni—P); a seed layer 12A of an amorphous or fine-grained material, e.g., a nickel-aluminum (Ni—Al) or chromium-titanium (Cr—Ti) alloy; a polycrystalline underlayer 12B, typically of Cr or a Cr-based alloy; a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (“DLC”); and a lubricant topcoat layer 15, e.g., of a perfluoropolyether. Each of layers 11-14 may be deposited by suitable physical vapor deposition (“PVD”) techniques, such as sputtering, and layer 15 is typically deposited by dipping or spraying.
In operation of medium 1, the magnetic layer 13 is locally magnetized by a write transducer, or write “head”, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer 13, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium layer 13 can subsequently produce an electrical response in a read transducer, or read “head”, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (SMNR) of the magnetic media. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, including that presented by the superparamagnetic limit, is to increase the crystalline anisotropy, thus the squareness of the magnetic bits, in order to compensate for the smaller grain sizes. However, this approach is limited by the field provided by the writing head.
Another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide a stacked layer structure wherein stabilization of a stacked pair if of ferromagnetic layers is provided via coupling of a ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer. In this regard, it has been recently proposed (E. N. Abarra et al., IEEE Conference on Magnetics, Toronto, April 2000) to provide stabilized magnetic recording media (hereinafter “AFC” media) comprised of at least a pair of ferromagnetic layers which are anti-ferromagnetically-coupled means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability.
The strength of the coupling between stacked ferromagnetic layers can be described in terms of the total exchange energy. For a pair of ferromagnetic layers separated by a non-magnetic spacer layer, the total exchange energy generally results from RKKY-type interaction (i.e., oscillation from anti-ferromagnetic to ferromagnetic with increasing spacer film thickness), dipole-dipole interactions between grains of the ferromagnetic layers across the spacer layer (which favors anti-ferromagnetic alignment of adjacent grains across the spacer layer), and direct exchange interaction (which favors ferromagnetic alignment of the ferromagnetic layers). In AFC media the thickness of the spacer layer is chosen to maximize the anti-ferromagnetic coupling between the ferromagnetic layers. According to this approach, the total exchange energy between the ferromagnetic layer pairs is a key parameter in determining the increase in stability.
Still another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization, hence increased SMNR, via formation of laminated media (hereinafter “LM”), as for example, disclosed in U.S. Pat. No. 5,051,289, the entire disclosure of which is incorporated herein by reference. Such LM comprise typically two or more stacked ferromagnetic layers separated by a non-magnetic spacer layer, where, in contrast to AFC media, the spacer layer generally is thicker and is provided for physically separating, rather than coupling, a pair of vertically stacked ferromagnetic layers; i.e., the strength of any magnetic coupling between the stacked ferromagnetic layers is smaller than the magnetic energies of the grains of each of the ferromagnetic layers.
It is also considered that improvement in the SMNR of longitudinal magnetic recording media required for providing a further increase in areal recording density cannot be obtained by further decrease in average grain volume (Vav.). The attainable value of the SMNR of magnetic recording media increases: (1) in approximate relation to N1/2, where N is the number of grains/recorded transition; and (2) with decreasing magnetic remanence-thickness product (Mrt) of the media. In either instance, however, the increase in SMNR leads to a smaller energy barrier (KuV) resisting magnetization reversal due to thermal agitation. The reduction in volume (V) can be partially offset by increasing the anisotropy (Ku) of the ferromagnetic material of the media; however, increase of the latter is limited by the strength of the currently available writing fields.
As indicated supra, different designs of the above-described LM and AFC media have been proposed for further increasing the areal recording density of longitudinal magnetic recording media. In LM, the number of grains/magnetic transition increases by a factor n, where n is the number of stacked ferromagnetic layers, such that the SMNR is expected to increase in approximate proportion to n1/2. However, obtaining stable LM with the requisite narrow width at one-half the peak height hereinafter “PW50”) for high areal recording densities is difficult with the currently available writing fields. On the other hand, in AFC media, stability of the main ferromagnetic recording layer increases: (1) due to the decrease in the demagnetization field upon storage which is proportional to the net Mrt, which is equal to the Mrt of the main layer (“ML”) minus the Mrt of the bottom AFC layer (“BL”), i.e., (Mrt)ML−(Mrt)BL and (2) due to the increase in the energy barrier due to coupling (H. J. Richter and E. Girt, submitted to Phys. Rev. Lett.), which stability increase of the main recording layer can be “traded off” against the decreased average grain volume (Vav.) in the main recording layer.
Notwithstanding the obtainment of improved thermal stability and/or SMNR afforded by each of the above-described AFC and LM approaches, the continuously increasing requirements for high storage density magnetic media exhibiting high SMNR and improved thermal stability necessitate an even further increase in the performance of such longitudinal-type magnetic recording media.
The present invention, therefore, addresses and solves problems attendant upon forming very high areal recording density, thermally stable, high SMNR longitudinal magnetic recording media, e.g., in the form of hard disks, which media combine the advantages afforded by both of the above-described AFC and LM approaches, while maintaining full compatibility with all aspects of conventional automated manufacturing technology therefor. Moreover, manufacture and implementation of the present invention can be obtained at a cost comparable to that of existing technology/methodology.